"M-i-s-t. It's a kind of veil separating the mortal world from the magic world. Mortal minds — they can't process strange stuff like gods and monsters, so the Mist bends reality. It makes mortals see things in a way they can understand — like their eyes might just skip over this valley completely, or they might look at that dragon and see a pile of cables." (4.73)
We never really find out if there is a Mist, or if it's just a metaphor. Do human minds reject magic automatically? Or is there some substance that erases it from their minds? It's not clear if the Mist that changes reality is real or an illusion.
"It was my fault," Piper said without thinking. She just couldn't stand it anymore. The secret about her father was heating up inside her like too much ambrosia. If she kept lying to her friends, she felt like she'd burn to ashes.
"Piper," Jason said gently, "you were asleep when Festus conked out. It couldn't be your fault."
"Yeah, you're just shaken up," Leo agreed. He didn't even try to make a joke at her expense. "You're in pain. Just rest." (22.64-66)
Piper believes that what she did in her dream affected reality and caused the dragon to crash. Leo and Jason, though, think that if she was asleep she couldn't have done anything—which seems odd since anytime any one of them goes to sleep, they have visions and get new powers or information. They should know how this reality works by now, including that dreams are pretty darn real. But maybe they're just shaken up by the dragon falling out of the sky.
Jason gave her a smile, though he looked kind of nervous. It was the exact expression he'd had on his face after he'd kissed her the first time, up on the Wilderness School dorm roof—that cute little scar on his lip curving into a crescent. The memory gave her a warm feeling. Then she remembered that the kiss had never really happened. (22.72)
The kiss never happened, it's true. But nothing in the book really happened since, you know, it's a fictitious book. Jason kissing Piper is as real as anything else that the novel describes. In fact, it's more real in some sense, since it admits it's not real. For that matter, Piper's relationship with Jason is something that could have happened—girls and boys do kiss. Flying on a dragon is much less real, even if we're supposed to pretend that it's more real.
Leo fiddled with his copper wires. He felt like an intruder. He shouldn't be listening to this, but it also made him feel like he was getting to know Jason for the first time—like maybe being here now made up for those four months as Wilderness School, when Leo had just imagined they had a friendship. (35.50)
Leo's listening to Thalia tell Jason about his past. He's learning Jason's real story here, just like the reader. The false history he had with Jason—the backstory that never was because it happened before the book—is replaced with a "real" backstory. Jason is only as real as what we know about him.
"Is the fortress always hanging there?" Piper asked. "How can people not notice it sitting on top of Pikes Peak?"
"The Mist," Thalia said. "Still, mortals do notice it indirectly. Some days, Pikes Peak looks purple. People say it's a trick of the light, but actually it's the color of Aeolus's palace, reflecting off the mountain face." (36.35-36)
Pikes Peak does have a purple glow, and the book offers a fanciful explanation for why that is, giving us a kind of fairy tale version of reality. If you visit Pikes Peak in the future, you can carry this version in your head when you see the purple light.
"It wasn't real. It never even happened. So why do I remember it so vividly?"
Aphrodite smiled. "Because you are my daughter, Piper. You see possibilities much more vividly than others. You see what could be. And it still might be—don't give up." (39.30-31)
It's not clear why being Aphrodite's daughter should allow you to see possibilities. Maybe Aphrodite means it lets Piper see possibilities for romance in particular? Either way, it seems like being able to imagine a version of reality is one way to help bring it about. Though I doubt you'll get to ride a dragon no matter how hard you imagine Festus.
"He's worked his whole life to deny the old stories about gods and spirits, yet he fears those stories might be real. He fears that he's shut off an important part of himself, and someday it will destroy him. Now he's been captured by a giant. He's living a nightmare. Even if he survives…if he has to spend the rest of his life with those memories, knowing that gods and spirits walk the earth, it will shatter him." (39.46)
Tristan has worked his whole life to deny the stories from his Cherokee heritage. The version of reality he's afraid of, in some sense, is that these stories are real, and that assimilating to white culture and being successful in Hollywood has made him lose touch with this important part of who he is. What stories do you distance yourself from?
He wished this were a Tristan McLean movie. Then Piper's dad would be faking unconsciousness. He'd untie his bonds and knock out the giant with some cleverly hidden anti-giant gas. Heroic music would start to play, and Tristan McLean would make his amazing escape, running away in slow motion while the mountainside exploded behind him. (41.50)
Leo is wishing he were in a movie—but he is in a movie. Or, sort of—he's in a fantasy-adventure book, which is pretty close, complete with an amazing escape, explosions, and superpowers. Leo is wishing for a version of reality that he doesn't yet know he's living.
"Monsters," her Dad said, a tear tracing his cheek. "I live in a world of monsters." (45.40)
Tristan really needed to get into another book, the kind where the strapping adventurer is the hero. In this book's reality, though, kids are the heroes and adults always have to be rescued. Drinking the potion to wipe his memory is a way of forgetting which book he's in, and getting back into a more comfortable reality.
Across the green, her cabinmates looked disappointed that they hadn't witnessed a kiss. They started cashing in their bets.
But that was all right. Piper was patient, and she could see lots of good possibilities.
"Let's go," she told Jason. "We've got adventures to plan. (52.103-105)
Piper is picking the version of reality she wants to come true. When she says, "we've got adventures to plan," she's referring not just to their quest, but to being girlfriend/boyfriend—a possibility that seems pretty probable at this point.